How Can I Detect the Active Font Used in a Web Page?

Detecting the Active Font in a Web Page

When designing web pages with diverse fonts, it becomes crucial to ensure that the desired font is rendered on the user's browser. This can be a challenge when multiple fonts are defined in the CSS.

One approach to addressing this issue is to determine which of the specified fonts is actually being used. By identifying the active font, you can take appropriate actions, such as displaying a download link for fonts that are not installed on the user's system.

Font Detection Technique

A common technique used for font detection involves measuring the width of rendered text using a specific font. If the measured width matches the expected width for the desired font, it indicates that the target font is present on the user's device. However, this method is less reliable for monospaced fonts.

Example Implementation

The following code snippet illustrates how this technique can be implemented:

<div>
<pre class="brush:php;toolbar:false">// Get the element's width with the desired font
var desiredWidth = document.getElementById("font-detector").offsetWidth;

// Set the element to use a fallback font
document.getElementById("font-detector").style.fontFamily = "monospace";

// Measure the width again using the fallback font
var fallbackWidth = document.getElementById("font-detector").offsetWidth;

// Check if the desired font is installed
if (desiredWidth === fallbackWidth) {
  // The desired font is not installed
} else {
  // The desired font is installed
}

Benefits and Limitations

This font detection technique is relatively simple to implement and can be used to dynamically adjust the page's behavior based on the availability of specific fonts. However, it may not be foolproof and can be influenced by factors such as browser settings or user-installed extensions that modify font rendering.